Digital technologies are reshaping how small and medium-sized enterprises design, deliver, and capture value. Yet SMEs do not experience digital transformation in the same way as large firms because they often operate with narrower financial margins, limited managerial bandwidth, weaker digital infrastructures, and fewer specialised capabilities. This systematic review examines the literature on digital business model innovation in SMEs from. It focuses on three interrelated themes: value creation mechanisms, resource constraints and capability barriers, and market scalability pathways. The findings show that SME digital business model innovation is not a single phenomenon but a family of related changes involving digital channels, platforms, data-driven services, digitalised customer interfaces, digitally enabled operations, and ecosystem participation. Across the literature, value creation is most often linked to efficiency gains, enhanced customer access, improved responsiveness, digital service augmentation, and new market reach. The review concludes that the field remains fragmented and insufficiently cumulative. More integrated research is needed to explain how SMEs convert limited resources into digital capabilities, how these capabilities reshape business models, and under what conditions digital business model innovation supports scalable and sustainable growth.